During recent years, a variety of different types of exercising machines have been developed for exercising and strengthening certain groups of muscles in the body. However, no real effort has been made to develop a machine which is particularly adapted for exercising the muscles in the lower back, and particularly no such machine has been developed which varies the resisting force against the muscles when the back is moved back and forth between a forwardly bent position and a position with the spine in a substantially straight position.
It has been proposed that the muscles in the lower back may be exercised by repeatedly performing a so-called "dead lift" of a barbell weight in which a person maintains the legs stiff and bends the back forwardly and lifts the weight from the floor while straightening the back and then again bends the back forwardly while lowering the barbell weight to the floor. In this type of exercise, the resistance to the lifting force is very high when the spine is bent forwardly and the resistance force is reduced to near zero when the spine is in the straight or upright position. In this upright position, the only force being applied is a compression force on the spine and no resistance force is being applied to the muscles of the lower back. Therefore, the resistance force applied to the muscles in the lower back is not properly varied throughout all positions of the exercising movement.
It has also been proposed that the lower back muscles be exercised by a "hyper-extension" type of exercise in which the person lays face down on an elevated bench or platform with the feet restrained and with the upper portion of the body extending outwardly beyond one end of the bench. This exercise begins with the upper portion of the body hanging downwardly from the bench or platform with the spine in a forwardly bent position and then the upper portion of the body is raised and moved upwardly to substantially a horizontal position so that the spine is substantially straight. The upper portion of the body is then lowered and moved back to the forwardly bent position. In this exercise, the resistance to movement of the muscles in the lower back is very low and practically no resistance force is provided when the back is in the forwardly bent position while maximum resistance force is provided at the end of the upward movement. Thus, this exercise does not provide the proper amount of variable resistance throughout the entire movement of the upper body. Also, a pulling or extension force is applied to the spine when the upper portion of the body is in a vertical downwardly extending position.